From Dean Willett, Precision BioLogic Inc. As I read the 11-8-17 comments from Dr. Elman a bit of a lightbulb went off in my head. I’m sure you’ve seen the various publications that demonstrate the effect of C reactive protein (CRP) on APTT assays. Here are: https://www.ncbi.nlm.nih.gov/pubmed/22224499; and https://biblio.ugent.be/publication/5838447/file/6839536. There have been some proficiency samples sent around in recent years that demonstrate this as well. It appears that the presence of CRP in otherwise normal plasma samples can prolong the APTT significantly in a dose dependent manner. In the Devreese study, PTT–LA exhibited such a response (as did other APTT reagents). I’m sure you can see what I’m getting at. This could possibly explain some of those “nuisance” borderline results or unexplained prolonged APTTs that correct slightly upon mixing due to the 50% dilution of CRP in the patient sample. Since the patient described had been diagnosed with a clot, it’s quite possible that CRP levels were elevated. A 50/50 mix would bring those CRP levels down.
Thanks to Dean for this reminder, and to Dave McGlasson, who appended the comment recommending immunoassays for anti-cardiolipin and anti-beta-2-glycoprotein 1 antibodies, both valuable markers of anti-phospholipid syndrome. This, along with the 12-week repeat, would help define the patient’s condition.
We saw just what Dean
We saw just what Dean mentioned in a 2014 proficiency survey from NASCOLA. The sample was negative for lupus anticoagulant, and enriched with CRP–we reported a false positive result in the DRVVT and Staclot LA. In response to this, we have added interpretive data to every patient result that is reported, stating that false positive results can be seen in patients with high levels of CRP.